NEUTRALITY OF THE TISSUES AND TISSUE-FLUIDS 281 



various samples of saliva having been brought to a common reaction 

 on the alkaline side of absolute neutrality (neutrality to phenolphtha- 

 lein), the quantity of acid is estimated, by direct titration, which is 

 necessary to bring the reaction of the fluid to an arbitrarily chosen 

 reaction on the acid side of neutrality (neutrality to paranitrophenol) . 

 The measurement is in fact analogous to that employed by Henderson 

 in estimating the power of different acids to maintain the neutrality 

 of their solutions. Of course, to obtain physiologically interpretable 

 results with blood-serum it would be necessary to carry out the titra- 

 tions under a standard partial pressure of carbon dioxide, for example 

 in a vessel filled with alveolar air. The choice of indicators is limited 

 when the fluid under investigation is even faintly tinged with color. 

 For example the faint yellowish tinge of blood-serum interferes with 

 the sharpness of the end-point with paranitrophenol. 



We thus see that by a variety of interlocking mechanisms, consisting 

 in every instance of dynamic equilibria, the tissue-fluids of the higher 

 animals, which are to their individual cells the external media in which 

 they live, are kept extraordinarily constant in concentration, composi- 

 tion of mineral constituents, and hydrogen ion concentration. 



The very great susceptibility of most of the chemical reactions 

 which are involved in life-phenomena to slight changes of reaction, 

 may very readily be seen to involve relative stability of reaction as a 

 requisite to the orderly performance of life-processes. It is in fact an 

 almost universal rule, in the words of Loeb, that " life-phenomena occur 

 in a neutral liquid. " The ocean which is the original home of life, is, 

 thanks to the presence of bicarbonates and phosphates, a "buffer"- 

 solution and nearly neutral in reaction despite the life which swarms 

 therein. According to Palitzsch the extreme variation in the hydrogen 

 ion concentration of the ocean is from 1.1 x 10~ 8 N to 0.45 x 10~ 8 N H + , 

 corresponding to an exceedingly faint alkalinity of the order of that 

 found in the blood of mammals. In a very few instances only does 

 life subsist in a medium which deviates far from neutrality. When 

 secreting gastric juice, in the absence of neutralizing substances, the 

 cells of the gastric mucosa are bathed on the side toward the lumen 

 of the stomach by a fluid which may attain an acidity or hydrogen ion 

 concentration due to hydrochloric acid of no less than one hundredth 

 normal, or ten thousand times the acidity which would correspond to 

 the alkalinity of pancreatic juice. The "salivary glands" of certain 

 carnivorous molluscs, which probably correspond in function to the 

 gastric glands in mammalia, similarly secrete an acid juice in which 

 the high hydrogen ion concentration is attributable to Sulphuric Acid. 



With such rare exceptions, exhibited only by highly specialized and 

 adapted cells, the immediate environment in which living matter 

 subsists is extremely invariable in certain physical characteristics, 

 and this invariability which is essential to the normal occurrence of 

 life-phenomena, is brought about through the interplay of unique 



